I am a socialist fifth-columnist working from within the Establishment to bring this country to financial ruin. Not really! I’m just married to one. Neither of us realised it until this week, when Nick Clegg announced that the Government is introducing free school meals for all children under eight. This policy is based on the recommendations of the School Food Plan – a blueprint for improving the quality and take-up of school food in England, written by my husband, Henry Dimbleby, and his partner, John Vincent.

It’s a brilliant plan, if I say so myself. And I would, having spent the past year helping them to write it, unpaid, often long into the night. But not everyone approves.

“Free school meals for all, regardless of background, is an expensive socialist nonsense,” tweeted the editor of these very Comment pages. Yikes. The Taxpayers’ Alliance dismissed it as a “gimmick”, while the Right-wing Institute of Economic Affairs said it would merely “subsidise the children of affluent families”.

This is the main argument against universal free school meals (UFSM): that, especially in straitened times, it makes no sense to pay for pupils who can afford to buy their own. Wouldn’t that money be better targeted at the poor, or given directly to families to spend as they see fit?

It sounds logical enough. But here’s the thing: people aren’t always logical. People – not least children – behave in ways that are subtle and often surprising.

We know quite a lot about what UFSM can do for children because of two pilot projects that ran from 2009 to 2011 in Durham and Newham, east London. All primary school children in these areas were given free lunches, and data collected to assess whether, and how, it affected their health and academic performance.

Some improvements were immediate. Take-up of meals soared from 50 per cent in both areas to 72 per cent for Newham and 85 per cent for Durham. The average midday meal improved significantly: consumption of sandwiches fell by 27 per cent, soft drinks by 16 per cent and crisps by 18 per cent, while consumption of vegetables rose by 23 per cent.

Even more impressive, perhaps, were the academic results. Children in the pilot areas quickly moved ahead of their peers elsewhere, by almost a term. UFSM did more to improve literacy levels than the nationwide introduction of a compulsory “literacy hour” in 1998.

This may be partly because UFSM solves the problem of feeding pupils who are “not quite poor enough”. These children come from low-income families who don’t quite qualify for free school meals, but can’t afford to pay for them – or to put together a good packed lunch.

Hungry children can’t concentrate. They fall behind in lessons and may become disruptive. Putting a proper meal in their bellies, therefore, is good for the whole class. Intriguingly, however, the children who benefited most from UFSM were the very poorest – those already eligible for free meals. They had nothing material to gain: yet extending the benefit to their wealthier peers dramatically improved their prospects.

Why should this be? Partly because children have incredibly acute social antennae. Being eligible for free school meals still carries a stigma. Most schools do their best to disguise the identity of FSM pupils, but children are ingenious at winkling out the truth. One secondary school pupil told us you can always tell the FSM kids because they choose the same meal every day: their payment cards restrict the choices they can make.

This kind of social discomfort is what makes children dread school. UFSM may not make it vanish altogether – but it helps. And it can do wonders for the broader culture of the school. Just as families benefit from eating together, so do schools. UFSM enables everyone to sit down and break bread together. In the schools that have tried it, teachers are more likely to eat with the pupils – something that, touchingly, children really enjoy.

Pupils learn not just to eat proper food, but to absorb other, less tangible goodies, such as table manners and the art of conversation. The dining room becomes the symbolic hub of the school, binding pupils and teachers together with the invisible threads of ritual. Every school in the pilot reported benefits to behaviour, morale and cohesion. It is this that feeds back into academic performance.

By now, you may be stroking your sceptical Tory chin and muttering about the cost of funding this experiment in cockle-warming. Fair enough: £600 million is a big cheque to write. But a lot less than £6 billion – the current cost to the taxpayer of treating illnesses caused by bad diet.

The fiscal benefits of creating a cleverer, better-educated nation may be harder to predict, or measure. But it’s a funny kind of Conservatism that pooh-poohs a cost‑saving measure proven to make children healthy, wealthy and wise.